The present disclosure relates to an image reading device that is incorporated in digital copiers, image scanners, etc. and that scans and reads a document, and to an image forming apparatus incorporating such an image reading device. More particularly, the present disclosure relates to an image reading device adopting a sheet through system in which an image is read while a document is being transported.
Some conventional image reading devices incorporated in multifunction peripherals and the like employing an electro-photographic process are provided with a document transport device (automatic document feeder) of a type that transports, for reading, a sheet-form document one sheet after another to a document placement stage, where the document is read, and then discharges the document from the document placement stage. In such an image reading device, two ways of reading can be performed, namely, a sheet through system and a document stationary system. In the sheet through system, with a document presser closed, a document is read while being automatically transported by a document transport device. In the document stationary system, a document is read while an optical system (scanning means) is moved, with a document presser opened and closed each time the document is read to permit one sheet after another of the document to be placed on a document placement stage (contact glass). In the former sheet through system, reading operation of a document is performed with the optical system inside an image reading device held at a predetermined image reading position without moving for scanning. On the other hand, in the latter document stationary system, reading operation is performed with the optical system moving for scanning.
Reading methods adopted in such an image reading device include a CIS (contact image sensor) sensor method, which uses a photoelectric conversion element called a CMOS (complementary metal-oxide-semiconductor) sensor, and a CCD (charge-coupled device) sensor method, which uses a charge-coupled device called a CCD sensor. The CIS sensor method does not require a mirror, which is required in the CCD sensor method; this advantageously helps make a scanner unit incorporating the CIS sensor thinner, and thus helps reduce its cost owing to the simple structure. On the other hand, the CIS sensor has a very small depth of field as compared with the CCD sensor; this inconveniently makes it difficult to read uniformly a book document and a document with an uneven surface.
In recent years, the mainstream of the above-described sheet through system has been a simultaneous two-side reading system in which the obverse and reverse sides of a Mo-sided document are read simultaneously by use of different reading modules in one document transport operation by a document transport device. The reverse-side reading module is fixed inside the document transport device; it reads the image of a document at a reading position while the document is transported by a transport roller arranged opposite the reading module.
As a reverse-side reading module, a CIS sensor having a small depth of field is typically used. Thus, the transport roller arranged opposite the reading module is required to transport a document while keeping it in close contact with a reading surface (contact glass) of the reading module, and is thus arranged such that the vertex of the circumferential surface of the transport roller (the point where the circumferential surface of the transport roller and the reading surface make contact with each other as seen from the axial direction of the transport roller) coincides with the reading position of the reading module.
Here, when the vertex of the circumferential surface of the transport roller deviates from the reading position, the document is transported in a state displaced from the reading surface of the reading module; this may inconveniently cause image defects as result from the document image being out of focus. Thus, an image reading device is known which is provided with a position adjustment means for adjusting the position of a contact image sensor with respect to a white platen roller (shading roller).
As a method for detecting the inclination of a document transport device with respect to an image reading portion, an image reading device is known which detects the inclination of a document transport means as a result of a scanning means reading two check patterns formed on a bottom face of the document transport device while remaining at rest at a position facing the bottom face of the document transport device across a contact glass. The two check patterns are each so formed, with respect to the transport direction, as to have an intersection in a central part thereof, be increasingly wide toward a first end side of the bottom face in the axial direction upstream from the intersection, and be increasingly wide toward a second end side which is opposite from the first end side downward from the intersection. The two check patterns are arranged in line symmetry on opposite outer sides in the direction perpendicular to an image reading region.